Caliciviruses, represented by the prototype Norwalk virus, are the major cause of epidemic acute gastroenteritis in humans. A major obstacle to the study of these medically important viruses is our inability to establish their growth in cell culture. The molecular mechanisms responsible for the fastidious nature of these viruses are not known. The primary focus of this project is to gain a better understanding of the replication strategies of cultivatable strains of Caliciviridae and apply this information to the formulation of a successful strategy for the cell culture adaptation of the noncultivatable viruses of this family. Feline calicivirus (FCV) was selected for an analysis of its replication in vitro because it grows efficiently in cultured, permissive cells. Experiments were initiated with FCV to study receptor binding, infectivity of viral RNA, mapping of gene products in the viral genome, protein processing, and the mechanisms responsible for host cell and cell type restriction in cell culture. Preliminary studies indicate that cell tropism of FCV involves functions subsequent to receptor binding, penetration and uncoating. Thus, FCV genomic RNA was infectious when transfected into a permissive cell line but infection did not occur when non-permissive cells were transfected. Clones overlapping the entire NV genome were generated and recombinant proteins were expressed in vitro. The functions of these proteins will be examined as the FCV protein functions are elucidated. In addition, extensive efforts will continue in an attempt to identify a cell strain or line in which Norwalk virus will replicate.